Quartz is commonly used as a lamp envelope material in metal halide lamps and tungsten halogen incandescent lamps. The quartz envelope defines a sealed lamp interior containing a filament or discharge electrodes and a suitable chemical fill. Electrical energy is supplied to the filament or to the electrodes by means of electrical feedthroughs which pass through the lamp envelope and are hermetically sealed to the quartz. It is critical to lamp operation that the seal remain intact throughout the life of the lamp.
It has been customary in quartz lamp envelopes to utilize a feedthrough configuration including a molybdenum ribbon, or foil, which passes through a press or pinch seal region of the quartz envelope. The molybdenum foil is sufficiently wide to conduct the required lamp current and is extremely thin. Since the molybdenum foil is very thin, its thermal expansion is extremely small. Thus, the probability of seal failure due to differential thermal expansion is small. In a conventional design, the quartz is press sealed to the molybdenum foil, and a molybdenum electrical lead is welded to the external end of the foil.
The molybdenum foil and the molybdenum electrical lead have a tendency to oxidize to form MoO.sub.2 and MoO.sub.3 molybdenum oxides. The molybdenum oxides initially form on the external electrical leads. The oxidation then progresses to the molybdenum foil and causes a significant amount of stress on the press seal. The stress is evident from Newton rings which appear at the point at which the leads are welded to the molybdenum foil. Eventually, the quartz press seal cracks, thereby causing the lamp to fail.
Various techniques have been utilized to limit molybdenum oxidation. One technique involves the deposition of a low melting glass frit at the end of the press seal where the electrical leads enter the press seal. The frit is intended to melt when the lamp is operating, thereby preventing oxidation from moving up the lead to the press seal. Occasionally, the frit melts and runs into the lamp socket, thereby causing additional problems. A high temperature melting glass frit has also been utilized. Neither frit is well suited for production and only slows the process of oxidation without stopping it.
In another prior technique, chromium is deposited on the molybdenum in a very high temperature pack cementation process. This is a very dangerous and inconvenient process. Pure hydrogen is passed through a tube furnace at 1200.degree. C. to initiate a reaction. The yield is very low, and devices are often damaged.
Various thin film coatings have been tried on the molybdenum with very little success. A major reason for the lack of success is that a coating of almost any thickness on the molybdenum foil causes added stress to the press seal and almost always leaves a path for oxidation to occur. Most coatings cannot withstand the temperatures encountered during fabrication of the quartz press seal. Many coatings melt or become uneven during operation and leave areas of exposed molybdenum which can become oxidized. Coatings can be used on the external electrical leads, since these leads do not form a hermetic seal with the quartz.
It is a general object of the present invention to provide improved quartz lamp assemblies.
It is another object of the present invention to provide quartz lamp assemblies having reliable, long-life press seals.
It is a further object of the present invention to provide quartz lamp assemblies with feedthrough components having oxidation-resistant surfaces.
It is still another object of the present invention to provide quartz lamp assemblies having oxidation-resistant molybdenum feedthrough foils.
It is yet another object of the present invention to provide quartz lamp assemblies with external molybdenum electrical leads having oxidation-resistant surfaces.